The invention relates to a method for adsorbing, on solid adsorbent particles, constituents dissolved in liquid or for desorbing (extracting), from solid desorbent particles, soluble constituents in liquid. The said constituents will often be organic in nature, but inorganic components, in particular heavy metals, are not excluded.
In trade and industry water is used in very large quantities, for example as a relatively inexpensive means of transportation, solvent, cleaning medium, coolant etc. The water often contains a range of dissolved substances which have been added deliberately in the form of base materials or adjuvants, or which have ended up therein as a result of reactions or inevitable contact with products or materials which may or may not have been contaminated. The discharge of water used for process purposes is allowed to take place only if the quality meets the antipollution requirements laid down by the authorities. These requirements are gradually being tightened. Consequently, there is an increasing demand, in various industrial sectors, for methods by means of which the process water and possible other liquids can be utilized more efficiently, or can be kept in use or circulation for longer periods.
A number of methods is known by means of which organic substances and/or inorganic substances can be removed or recovered selectively from aqueous mixtures. Examples include solvent extraction, distillation, stripping, filtration using diatomaceous earth, adsorption on activated carbon, chemical oxidation, membrane separation processes and biological processes. These methods have limited applicability and/or limited selectivity or have other drawbacks such as high energy consumption, secondary emissions or excessive costs for treating small volumes.
Selective adsorption with the aid of adsorption chromatography might provide a way out, albeit that this method has serious drawbacks. Owing to batchwise processing, the required amounts of adsorbent often constitute the limiting factor. Furthermore, many process water streams have high flow rates, which means that the use of large adsorption columns is undesirable on financial grounds.
Also used on a large scale in trade and industry is solids extraction, inter alia for producing edible oils from oleiferous seeds, producing instant coffee or tea, leaching ores, treating of contaminated sludges, producing valuable water- or alcohol-soluble components from a large range of raw materials, treating/washing particles. To this end, continuous apparatuses are often used, based on the percolation or immersion principle, such as the so-called Rotocel extractor (a segmented carousel), perforated-belt/compartmented extractors, rotating-disc extractors, screw extractors etc. These methods all have the drawback that they can not, or not effectively, be used with small particles to be extracted (smaller than 200 xcexcm), particularly if these are compressible and have a density which does not greatly exceed that of the extraction fluids. Packed percolation beds then become clogged and/or insufficient separation takes place between the particles and the liquid, as a result of which good, rapid countercurrent extraction is not obtained. On the other hand, the use of small particles is desirable precisely because the factors impeding diffusion are greatly restricted thereby and it would be possible to perform the extraction much more rapidly and/or at a lower, milder temperature.
It is an object of the invention to provide a method as indicated in the preamble, wherein the abovementioned drawbacks are avoided.
According to the invention, the method is characterized in that in a container a packed bed is formed from granular material, mixed with adsorbent particles or desorbent particles smaller than 200 xcexcm, in that said bed moves downwards by virtue of gravity, in that said liquid is directed upwards through the bed by means of a pressure differential, said dissolved constituents being adsorbed on the solid adsorbent particles or the soluble constituents being extracted from the desorbent particles.
The adsorbent particles or desorbent particles are introduced into a bed of coarser granular material, as a result of which even small, compressible and/or light particles are completely enclosed in the granular-material bed and this bed is discharged downwards as a packed bed, coming into countercurrent contact with a liquid flowing upwards, the result being a countercurrent adsorption chromatography process or a countercurrent extraction process. These processes, now that small particles can be used, will require a much shorter contact time than the processes known hitherto, as a result of which the throughput of the liquid to be treated or the throughput of the particles to be treated by means of extraction can be greater than is customary in known processes. It may be possible for extraction processes to take place at milder process conditions, as a result of which the product quality can increase. Equally, the residence time can be shortened and operations can, for example, be carried out at a lower temperature. Furthermore it may be possible to use environment-friendly extraction fluids. In the case of adsorption chromatography there is the option for different adsorbent particles to be used simultaneously, as a result of which the selectivity of various dissolved constituents to be removed or to be extracted can be increased. Furthermore, the process can be combined with the removal of undissolved constituents from the liquid stream to be treated, which is a known use of moving packed beds of granular material. In that case the adsorbent particles should be chosen so as to allow simple separation between particles and undissolved constituents to be removed (lamellar settling, hydrocycloning, flotation etc.).
The granular material used will normally be sand having a diameter of between 0.3 and 5 mm; other granular materials (glass, metal, plastic) can also be used, however. Its function is not to filter the influent but to effect downward transport, countercurrent to the liquid to be treated, of the small particles which, because of the sand, essentially maintain their position. The countercurrent of the liquid and the particles in particular results in high efficiency being achieved.
The packed bed can comprise the following sections:
a top section which comprises inert granular material and imparts stability to the bed,
an adsorbent or desorbent middle section comprising inert granular material mixed with adsorbent particles or desorbent particles, and
a bottom section which is to be discharged and comprises inert granular material and laden adsorbent particles or desorbent particles.
Preferably, the adsorbent particles or desorbent particles are fed in approximately between the top and middle bed section in a concentrated slurry by means of an injection distribution means and the influent is introduced approximately between the bottom and middle bed section and the effluent is removed from the top bed section.
Preferably, granular material containing the adsorbent particles or desorbent particles is transported from the bottom bed section to a point above the top bed section, the granular material and the adsorbent particles or desorbent particles are subsequently separated from one another, the separated-off granular material is passed into the top bed section and the separated-off adsorbent particles or desorbent particles are discharged.
The transport of the adsorbent particles or desorbent particles to a point above the top bed section can make use, for example, of a gas-lift pump effect, which involves a pressurized gas being passed into the bottom end of a central pipe. Other transport means are also possible, however, (screw conveyor, slurry pump, conveyor belt and the like). The riser could also be positioned outside the tank.
If the method is used for adsorbing, on solid particles, constituents dissolved in liquid, the laden adsorbent particles are regenerated and recycled into the process.
Preferably, the laden adsorbent particles are concentrated prior to being regenerated.
The regeneration in this context can take place by means of desorption with the aid of an acid, alkali or solvent, by an increase in temperature or with the aid of catalytic oxidation using ozone, peroxide or some other oxidant.
The invention also relates to an apparatus for bringing small solid particles in a liquid stream into contact countercurrently, in particular for implementing the method according to any one of the preceding claims, characterized by
a column containing a packed bed comprising three layers whose bottom two layers comprise granular material with small adsorbent particles or desorbent particles trapped in between and the top layer comprises a covering layer of granular material,
means for discharging the bottom layer, as a result of which the packed bed moves slowly downwards at a controlled velocity,
means for injecting and distributing a liquid between the bottom and middle layer and for discharging said liquid above the top layer,
means for injecting, between the middle and top layer, a concentrated slurry dispersion of small particles and distributing it in such a way that these particles are entrained through the downward-moving packed bed, countercurrent to a liquid flowing upwards,
means for separating the discharged slurry, which comprises granular material and small particles forming the bottom layer, into granular material and small particles.
Such an apparatus can be used quickly, since existing equipment for moving-bed sand filtration can easily be adapted.
The invention will now be explained in more detail with reference to the FIGURE.